Abstract: In recent five years, great breakthroughs have been made successively in the Lower Triassic Baikouquan formation in hydrocarbon?rich Mahu sag in the northwestern margin of Junggar basin, and the above?source quasi?continuous oil reservoirs with the reserves over hundreds of millions of tons are firstly discovered, which should be attributed to the changes of thoughtway from structural reservoir exploration to lithological?stratigraphical reservoir and continuous reservoir exploration. Composite analysis on hydrocarbon accumulation mechanism shows that high?quality Carboniferous?Permian source rocks are developed in hydrocarbon?rich Mahu sag, which is favorable for oil generation; high?quality reservoirs and structural?lighological traps of fan?delta front facies are developed in Baikouquan formation, in which faults connect the source rock and the reservoir, which helps hydrocarbon accumulation; gentle structures with good top and bottom conditions in Baikouquan formation are in favor of hydrocarbon enrichment; abnormally high pressure and fractures occur in Baikouquan formation, and the oil here is of good quality and reservoir sweet spots are developed, which is good for high oil production. Therefore, quasicontinuous reservoirs with the reserves of hundreds of millions of tons in Baikouquan formation in Mahu sag are characterized by extensive oil distribution, local oil enrichment, jointly controlled by nose?like structural belts and favorable belts in delta front as well as faults, high?efficiency accumulation and great prospecting potential, which can be an important supplement to continuous?reservoir exploration and research in the world. It is suggested that Huangyangquan fan, Madong fan and Xiayan fan with relatively low exploration degree should be focused on for further petroleum exploration at present